This invention relates to making electrical measurements with low level ac signals, e.g., in the microampere and microvolt ranges, useful, e.g., in testing ICs on circuit boards.
In a U.S. patent application entitled "Analyzing Electrical Circuit Boards" recently filed by Joseph Wrinn and Mark Hoffman, Ser. No. 880,589, filed Feb. 23, 1978 (hereby incorporated by reference), there is disclosed Wrinn's and Hoffman's joint invention in which the internal resistance of ICs is determined using signal of sufficiently low level not to disturb normal operation of a powered up board, with signal processing circuitry in which a three tip probe is transformer coupled to a synchronous detector.